Heat treatable tool steel of high carbide content



y 11, 1965' E. GREGORY ETAL 3,183,127

HEAT TREATABLE TOOL STEEL OF HIGH CARBIDE CONTENT Original Film April27. 1959 Flip/74" Amway W4/7'5 WP/TE k answer (mas/a5 MPM/KY/IE fi Pwmer(42am: CZC

INVENTORS maer a /[4,? tin/9 ATTORNEY.

3,103,112? HEAT TREATABLE TOOL STEEL OF HIGH CARBIDE CONTENT EricGregory, Bernardsville, N..l., and Martin Epner, Yonkers, N.Y.,assignors, by mesne assignments, to Chromalloy Corporation, New York,N.Y., a corporation of New York Original application Apr. 27, 1959, Ser.No. 809,217. Divided and this application Apr. 30, 1963, Ser. No.

This application is a division of our US. application Serial No.809.217, filed April 27, 1959, now abandoned.

This invention relates to carbidic steels and in particular to a heattreated tool steel containing as a primary carbide at least one carbideselected from the group consisting of VC, CbC and TaC.

In US. Patent No. 2,828,202 dated March 25, 1958, and issued to the sameassignee, a tool steel of high carbon content based on titanium carbideis disclosed in which the amount of titanium employed is at leastsubstantially all combined in the form of a primary carbide. Thetitanium carbide is uniformly distributed through a heat treatableferrous matrix comprising either carbon steel, medium alloy steel orhigh alloy steel.

As pointed out in the aforementioned patent. the composition is formedby employing titanium and carbon together in a combined form as titaniumcarbide as an alloying ingredient together with a steel matrix whichcooperates with said carbide in producing the desired composition. Thesteel employed in forming the matrix contains iron as the major alloyingelement which generally comprises at least about 60% by Weight of thesteel matrix composition. The amount of titanium may range from about10% to 70% by weight (about to 90% by volume of titanium carbide or12.5% to 87% by weight) and preferably'about20% to 58% by weight oftitanium (about'40% to 80% by volume of titanium carbide or to 75% byweight), substantially the balance being formed of the steel matrix.

Powder metallurgy is employed as the preferred method of producing thedesired composition which comprises broadly mixing powdered titaniumcarbide with powdered steel-forming ingredients and forming a slug bypressing the mixture in a mold, followed by subjecting the slug toliquid phase sintering under non-oxidizing conditions such as in avacuum.

We find the foregoing method best adapted to produce the tool steel ofthe invention, particularly where the amount of primary carbide is ofthe order of about to 60% by weight of the total composition. for thereason that the formation of large dendritic carbides are preventedwhich otherwise would occur if the elements V. Cb or Ta and carbon wereadded to a molten bath of iron such as is done in the addition oftungsten and chromium in the making of high speed steel.

It would be desirable to have a product with substantially large amountsof the primary carbide and yet be capable of being heat treated over awide range of hardnesses without change in shapeof the primary carbidegrain. By primary carbide is meant that carbide which is substantiallyunaffected by normal steel heat treating practices. Of particularimportance would be ahighly carbidic product capable of being annealedto relatively low hardnesses for machining and other purposes.

We have found that tool steel compositions containing 5 Claims.

substantial amounts of CbC or TaC, e.g. as high as 45 weight percent ofthe carbide are capable of being annealed to as low as 30 to Rockwell Cand hardened to as high as 65 to 70 Rockwell C." This decrease inannealed hardness, compared to R obtained with cer- Patented liviiagrill, 11065 tain of the titanium carbide tool steel compositions, isimportant as it greatly improves the machinability of the steel alloyand extends its use to the manufacture of complicated shapes where highhardness in the heat treated condition is an essential requirement.

It is the object of the present invention to provide a new tool steelcomposition containing substantial amounts of a primary carbide and yetwhich can be heat treated. e.g. annealed and hardened similar toconventional tool steels.

Another object is to provide as an article of manufacture a tool steelbar stock containing substantially large amounts of carbon and a metalfrom the group consisting of V, Cb and Ta in the form of sharplycornered primary carbide grains distributed uniformly through a heattreatable steel matrix.

A still further object is to provide a carbidic steel which in thehardened condition has to a large extent the attributes of cementedcarbides.

'These and other objects and advantages will become apparent from thefollowing description taken in conjunction with the accompanying drawingwherein:

FIG. 1 depicts a representation of a photograph taken at 1000 diametersshowing a microstructure comprising primary grains of columbium carbidedistributed through a pearlitic steel matrix;

FIG. 2 is similar to FIG. 1 but shows primary grains of tantalum carbidedistributed through a pearlitic steel matrix: and

FIG. 3 depicts a representation of a photomicrograph taken at 1000diameters showing primary columbium carbide grains distributed through amartensitic steel matrix.

In producing the carbidic tool steel of the invention, the primarycarbide selected from the group consisting of VC, CbC and TaC may rangefrom about 15% to 90% by weight (about 20% to 90% by volume),substantially the balance being the steel matrix. Preferably, thecarbide composition may range from about 20% to by weight (about 35% toby volume) and the balance steel.

The steel employed in the invention as the ferrous matrix contains ironas the major alloying element, which generally comprises at least about60% by weight of the matrix composition. Thus, in carrying out theinvention. the steel may be an alloy steel, a carbon steel or maycomprise pure iron to which carbon is added to form the required steelcomposition. The carbon present, exclusive of that in the primarycarbide. should be sufficient to confer heat treatability to the ferrousmatrix.

The expressions ferrous matrix or steel matrix as employed herein is onewhich crystallographically at ordinary temperatures is characterized inthe annealed state by a substantiall ferritic or body centered cubicstructure and which at an elevated temperature below the melting pointof the ferrous alloy is transferred to a substantially austenitic orface-centered cubic structure.

As illustrative of the useful compositions provided by the invention,the following examples are given:

Example I A heat treatable steel containing substantial amounts of CbCwas formed by using the following ingredients:

Columbium carbide:

Percent Cb About 86.0

Percent Ta About 2.0

Percent Ti About 0.6 Percent total Carbon About 10.72 Percent freecarbon About 0.05 Percent Fe About 0.1 Percent Si About 0.03 Percent CaAbout 0.03

3 This powder had an average particle size based on the Fisher sub-sievesizer of about 8.71 microns.

The iron powder employed in producing the steel matrix comprisedcarbonyl iron powder containing the following:

Percent Fe 99.6 to 99.9 Percent C 0.01 to 0.06 Percent O 0.10 to 0.30Percent N 0.00 to 0.05

The average particle size was 20 microns.

In producing the steel composition, a mixture containing by weight 45%CbC, 54.72% Fe and 0.28% C was prepared with one gram of parafiin waxfor each 100 grams of mix by ball milling the ingredients for 60 hoursin a stainless steel ball mill half filled with stainless steel ballsusing hexane as a vehicle. After milling, the mixture was dried on a hotplate at 150 F. until all the hexane was driven ofi. The dry powder waspressed into briquettes or slugs at tons per square inch.

The briquettes were then subjected to sintering by heating them to 1450"C. in 2 /2 hours in vacuum, holding at temperature for three-quarters ofan hour. followed by cooling to 1300" C. in 30 minutes and then from1300 to room temperature by furnace cooling. The sintcring was carriedout on a ceramic plate of previously fired Magnorite (a commercial MgOrefractory). The sintered briquettes were then annealed by heating atl575 F. for 2 hours followed by cooling from 1575 F. to 1300 F. at therate of /hour in hydrogen and then allowed thereafter to furnace cool toroom temperature. The as-sintered hardness of the steel was 39.1 R whichdropped to 34.7 R after annealing. condition, the steel had a modulus ofrupture of about 226,000 p.s.i.

The annealed steel is then hardened by austenitizing at 950 C. for onequarter hour and quenched in oil or water. When oil quenched thehardness was 67.4 k while water quenching 'gave a value of 690 R Thesteel had a density of 7.81 grams per cubic centimeter.

The microstructure of the foregoing composition in the annealedcondition is shown in FIG. 1 as comprising primary carbide grains of CbCdistributed through a steel matrix containing pearlite.

FIG. 2 shows a microstructure of the steel of Example 1 in the hardenedstate as comprising primary carbide grains of CbC distributed through asteel matrix contain ing martcnsite.

Example 2 A heat treatable steel based on a primary carbide of TaC wasproduced from the following ingredients:

Tantalum carbide:

Percent Ta and Ch About 93.3 to 93.7.

f Percent'Cb Up-to 1 (included above).' Percent total carbon About 6.22.

Prcent free carbon About 0.11.

Percent Si0 (max.) 0.05.

Percent Ti (max.) 0.2.

Percent CaO (max.) 0.2.

The particle size based on the Fisher sub-sieve sizer was 4.18 microns.The iron used in forming the matrix was the same as that used inpreparing the steel alloy of Example 1. v

In producing the steel composition, a mixture containing by weight 56.2%TaC, 43.58% Fe and 0.22% C as carbon black was prepared in the samemanner as described in Example 1. The same technique was used inpressing and sintering the composition. The as-sintered hardness was ofthe order of about 29.1 R and asannealed was slightly higher at 30.4 RThe oil quenched hardness using the same heat treatment desribed inExample 1 was about 65.0 while the water quenched hardness was 66.4 Rstate was 207,000 p.s.i., the density being 10.46 grams/cc.

The transverse rupture in the as-sintered In the as-sintered Percent\r'C. Percent (.bt? lt-rcont Tat) Peri-rut Fe Percent (I It will benoted that assuming no carbon leaves the steel matrix the carbon contentof the matrix will calculate to about 0.5%.

As has been stated, generally the composition will comprise about 15% to90% by weight of a carbide selected from the group consisting of VC,CbC, and TaC with the balance being made up of a steel matrix.Preferably, the composition will comprise about 20 to by weight of theprimary. carbide.

While steel alloys 3 to 8 above are substantially based on a matrix of0.5 carbon steel, it is understood that a wide range of steelcompositions may be employed. These include SAE 1010 steel, SAE 1020steel, SAE 1030 steel, SAE 1040 steel, 'SAE 1080 steel, etc. Pure ironmay be used since it combines with carbon to form a steel during theprocess of producing the ferrous alloy of the invention. Low, medium andhigh alloy steels may also be employed, including the following: about0.8% chromium, 0.2% molybdenum, about 0.30% carbon, and ironsubstantially the balance; about 5% chromium, 1.4% molybdenum, 1.4%tungsten, 0.45% vanadium, 0.35% carbon, and iron substantially thebalance; about 8% molybdenum, 4% chromium, 2% vanadium, 0.85% carbon,and iron substantially the balance; about 18% tungsten, 4% chromium, 1%vanadium, 0.75% carbon, and iron substantially the balance; about 20%tungsten, 12% cobalt,- 4% chromium, 2% vanadium, 0.80% carbon and ironsubstantially the balance; and generally other types of steelscharacterized'crystallographically by a body centered cubic structure atordinary temperatures and by being transformable to a face centeredcubic structure at an elevated temperature below the melting point ofthe steel. 9

One of the main advantages of the carbidic steel provided by theinvention is that it is possible to produce a heat treatable productcontaining large amounts of a primary carbide in the form of isolated,sharply angled grains distributed through a steel matrix without formingmassive carbide dendrites. This is achieved by utilizing powdermetallurgy as described hereinabove. Another method that can be employedcomprises coalescing primary carbide grains into a coherent porous bodyby briquetting followed by firing at an elevated temperature, generallyfrom about 1000" C. to 1600 C. for about one half hour to six hours,preferably at a vacuum or a subatmospheric pressure not exceeding about300 microns of mercury. As an alternative method, the coalescing may beachieved by simultaneously briquetting-and firing at the indicatedtemperature range under non-oxidizing conditions for about ten minutesto about two hours. The first porous body is then prepared for thecasting process by encasing it in a mold of refractory materialsubstantially inert to the ferrous alloy, for example stabilizedzirconia, with provisions made for the molten steel to enter the moldand contact the porous carbide structure. The' mold of refractorymaterial and the porous carbide structure supported therein is thenplaced into a suitable casting furnace. Sufficient amount of steel toproduce the casting is placed at the mold opening and the whole broughtto a temperature of generally up to about 100 C. above the melting pointof the steel so that the molten steel flows interstitially into theporous body, completely filling it and providing excess feed forshrinkage cavities, pipes, etc. The casting is achieved in vacuum or ata sub-atmospheric pressure generally not exceeding about 300 microns ofmercury. After the steel has interstitially filled all of the voids inthe porous primary carbide structure, and then allowed to reachequilibrium with it, the carbide is modified by partial solution in theliquid phase, whereby it is disrupted into discrete and uniformlydistributed grains. The interstitially cast ferrous alloy body is cooledin vacuum, is re moved from the furnace and is finally separated fromthe refractory mold. Thereafter the product is subjected to annealingand heat treatment.

We prefer to use the method of mixing the ingredients together, forminga shape thereof and sintering the shape at an elevated temperature for atime sutlicicnt to obtain substantially full densification. Broadly thismethod comprises mixing the appropriate amount of steel-formingingredients with the appropriate amount of the primary carbide, using asmall amount of wax to give sufficient green strength to the resultingpressed compact, for example one gram of wax for each 100 grams ofmixture. The mixture may be shaped a variety of ways. We prefer to pressthe mixture to a density at least 50% of true density by pressing overthe range of about t.s.i. to 75 t.s.i., preferably t.s.i. to 50 t.s.i.,followed by sintering in a vacuum, preferably below 300 microns ofmercury, preferably at a temperature above the melting point of thesteel matrix, depending on the alloying ingredients present, rangingfrom about 1300? C. to l575 C. for a time sutficient for the primarycarbide and the matrix to reach equilibrium and to obtain substantiallycomplete densification, for example for about one minute to six hours.

When the liquid phase sintering is completed, the product is allowed tofurnace cool to room temperature. If necessary, the as-sintered productis subject to any me chanical cleaning and then annealed at atemperature in the range of about 650 C. to 975 C. for about V1 hour to4 hours and then cooled at a rate of l0 C./hour to below 600 C., andthereafter furnace cooled.

The annealed structure will generally show a microstructure of primarycarbides distributed substantially uniformly through a steel matrixcomprising an austentitie decomposition product such as pearlite andferrite, or, depending on the heating cycle, spheroidite and ferrite.

The hardening is achieved by heating to an austenitizing temperature,e.g. in the range of about 870 C. to l3l5 C. for a time sufficient toconvert substantially the matrix to a face centered cubic structure,e.g. one minute to three hours, and then subsequently quenched bycooling in air, oil or water, depending upon the composition of theferrous alloy, thus decomposing austenite tomartensite. The austenitemay also be transformed into bainite by isothermally quenching to abainite formation temperature from the aforementioned austenitizingtemperature.

The primary carbides of the group VC, CbC and TaC may include limitedamounts by weight of other carbides,

such as up to about 50% tungsten carbide, up to about 50% molybdenumcarbide, up to about 10% chromium carbide, up to about 25% zirconiumcarbide, up to about 25% titanium carbide, and the like. The totalamounts of other carbides will generally range up to 50% by Weight ofthe primary carbides present.

Generally, as heat treated, the alloy of the invention will have amicrostructure comprising any one of the austenitic decompositionproducts pearlite, bainite and martensite.

The invention provides a carbidic heat treatable ferrous alloy which inthe form of bar stock, rounds, squares, blocks, ingots and other shapescan be utilized in the fabrication of cutting tools, blanking dies,forming dies, drawing dies, rolls, hot extrusion dies, forging dies,upsetting dies, broaching tools and in general all types of wear and/orheat resisting elments, tools or machine parts.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the in-- vention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What is claimed is:

l. A hardened, wear resistant, high carbon tool steel consistingessentially of about 15% to 90% by weight of primary carbide based on atleast one carbide selected from the group consisting of VC, CbC and TaCdistributed substantially uniformly through a ferrous matrix consistingessentially of the balance, said ferrous matrix containing iron as themajor alloying constituent and containing combined carbon and consistingessentially of a microstructure selected from the group consisting ofmartensite and bainite.

2. A hardened, wear-resistant, high carbon tool steel consistingessentially of about 15% to 90% by weight of a primary carbide based onat least one carbide selected from the group consisting of VC, CbC andTaC distributed substantially uniformly through a ferrous matrixconsisting essentially of the balance, said ferrous matrix containingiron as the major alloying constituent and containing combined carbonand consisting essentially of a microstructure of martensite.

3. A hardened, wear-resistant, high carbon tool steel consistingessentially of about 20% to by weight of a primary carbide selected fromthe group consisting of VC, CbC and TaC distributed substantiallyuniformly as sharply cornered grains through a ferrous matrix consistingessentially of the balance, said ferrous matrix containing iron as themajor alloying constituent and containing combined carbon and consistingessentially of a microstructure of martensite.

4. A hardened, wear-resistant, high carbon tool steel consistingessentially of about 15% to by weight of a primary carbide selected fromthe group consisting of VC, CbC and TaC distributed substantiallyuniformly through a ferrous matrix consisting essentially of thebalance, said ferrous matrix containing iron as the major alloyingconstituent and containing combined carbon and consisting essentially ofa microstructure of bainite.

5. A hardened, wear-resistant, high carbon tool steel consistingessentially of about 20% to 75% by weight of a primary carbide selectedfrom the group consisting of VC, CbC and TaC distributed substantiallyuniformly as sharply cornered grains through a ferrous matrix consistingessentially of the balance, said ferrous matrix containing iron as themajor alloying constituent and containing combined carbon and consistingessentially of a microstructure of bainite.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK,Primary Examiner.

1. A HARDENED, WEAR RESISTANT, HIGH CARBON TOOL STEEL CONSISTINGESSENTIALLY OF ABOUT 15% TO 90% BY WEIGHT OF PRIMARY CARBIDE BASED ON ATLEAST ONE CARBIDE SELECTED FROM THE GROUP CONSISTING OF VC, CBC AND TACDISTRIBUTED SUBSTANTIALLY UNIFORMLY THROUGH A FERROUS MATRIX CONSISTINGESSENTIALLY OF THE BALANCE, SAID FERROUS MATRIX CONTAINING IRON AS THEMAJOR ALLOYING CONSTITUENT AND CONTAINING COMBINED CARBON AND CONSISTINGESSENTIALLY OF A MICROSTRUCTURE SELECTED FROM THE GROUP CONSISTING OFMARTENSITE AND BAINITE.